An optical isolator is an optical element that enables light to be transmitted in one direction and disables the light to be transmitted in a reverse direction. For example, after the optical isolator is disposed on an emission end of a semiconductor laser, light emitted from the laser passes through the optical isolator and can be used as a light source used for optical communications. On the contrary, light that is about to be incident to the semiconductor laser through the optical isolator is stopped by the optical isolator, and cannot be incident to the semiconductor laser. If the optical isolator is not disposed on the emission end of the semiconductor laser, reflected light is incident to the semiconductor laser, causing that an oscillation characteristic of the semiconductor laser deteriorates; output intensity varies (generating an intensity noise); an oscillation wavelength changes (generating a phase noise); and the like.
Not only for the semiconductor laser, but also for an active element such as an optical amplifier, unexpected light reflection also causes degradation of working characteristics of the active element. To avoid this phenomenon, the optical isolator needs to be configured on an output end of the semiconductor laser or the active element to avoid occurrence of the undesirable phenomenon in the semiconductor laser or the active element. Especially, when the semiconductor laser is used as a light source used for high-speed optical fiber communications, oscillation stability of the light source is an absolute condition. Therefore, the optical isolator must be used.
Currently, most of optical isolators on the market are classical optical isolators, for example, Faraday magnetic-optic rotation crystal-type optical isolators and birefringence crystal-type optical isolators, and these classical optical isolators are used for encapsulation of a coaxial optoelectronic device. Compared with an optoelectronic device such as a planar lightwave circuit (PLC), the classical optical isolators are not easily integrated or encapsulated for main reasons that sizes of the classical optical isolators are relatively large and the classical optical isolators need to be placed perpendicular to a light propagation direction, which increases an encapsulation size of a device. Moreover, materials of the classical optical isolators are different from those of PLC optoelectronic devices and have more insertion losses.